In the field of sheet metal duct systems, round angle flanges are an important item, being used to connect sections of duct as reinforcements and to enable attachment of hangers. Previously known commercially available ring flanges, particularly in the larger diameters, are over-designed for sheet metal ductwork. Many are made from angle iron which is rolled into a helix, cut and welded. Others are made from angle iron which is cut, rolled and welded, while other angle ring flanges, particularly small sizes, are stamped and formed in a single stamping operation. Another method which has been attempted is to roll form a single angle ring flange from a flat strip of sheet metal. To an extent this latter attempt has been successful on larger diameter heavy gauge flange rings but even so the resulting standing flange had a curvature and when lighter gauge sheet metal and small diameters were used the single flange would slip sideways in the rolls resulting in a ruined angle ring flange.
The method of the present invention was developed to make lightweight angle ring flanges from lightweight formable sheet metal to avoid the unnecessarily heavy gauge ring flanges previously in common use. This has been a highly desirable development in that it decreases the weight of the assembled duct system as well as decreasing the cost of the base material. The lightweight angle ring flanges could not be satisfactorily made by single angle forming rolls so the present method of roll-forming a channel shape, effectively two joined angle ring flanges, and cutting the channel in half was developed and to avoid the necessity of two separate operations a special set of forming rolls was developed to enable finish forming the dual flange channel form and then shear cutting the channel in one continuous rolling operation and the unique set of rolls was developed to result in a clean shearing action which avoids a sharp edge offset along the sheared edge.
The prior art discloses various aspects of roll forming annular members with various contours from strip blanks which are rolled and welded to provide an annular flat band of sheet metal which is then formed through further steps to a final ring shaped or circular shaped item. Such prior art processes can be seen in the following U.S. Pat. Nos.: F. R. Klaus 1,710,930 issued Apr. 30, 1929 and J. M. Hothersall 2,139,682 issued Dec. 13, 1938 disclose methods of roll forming an annular blank into a double contour ring shape and without removing the shaped form from the rolls, cut or shear the form into two substantially identical items. Both patents disclose that further subsequent forming steps are performed on the severed items. E. A. Nelson U.S. Pat. No. 2,095,343 issued Oct. 12, 1937; A. S. Van Halteren U.S. Pat. No. 2,112,697 issued Mar. 29, 1938; C. W. Sinclair U.S. Pat. No. 2,151,568 issued Mar. 21, 1939; N. C. Rendleman U.S. Pat. No. 2,247,002 issued June 24, 1941; A. S. Van Halteren et al U.S. Pat. No. 2,316,029 issued Apr. 6, 1943; and E. G. Spisak U.S. Pat. No. 4,068,362 issued Jan. 17, 1978 disclose methods of roll forming an annular blank into a double contour circular shape form and subsequently performing a separate operation to cut or saw the form into two substantially identical pieces. Other patents which disclose a method starting with a flat strip blank, then rolling, welding and form shaping the blank to a single finished item are O. M. Whitten U.S. Pat. No. 2,200,569, issued May 14, 1940; V. Fincl et al U.S. Pat. No. 3,862,563 issued Jan. 28, 1975; E. G. Spisak U.S. Pat. No. 4,084,526 issued Apr. 18, 1978 and W. Bosch U.S. Pat. No. 4,143,533 issued Mar. 13, 1979.